Various printing plates for lithography using a silicone rubber or fluorine resin as the ink repellent layer without using any dampening water, especially selectively exposed and developed photosensitive lithographic printing plates have been being proposed.
For example, as positive type photosensitive lithographic printing plates, Japanese Patent Publication (Koho) SHO 54-26923 and 56-23150, etc. propose water-less lithographic printing plates with a photo polymerizable adhesive layer and a silicone rubber layer as an ink repellent layer laminated on a substrate, and Japanese Patent Publication (Koho) HEI 3-56622, Japanese Patent Laid-Open (Kokai) SHO 61-153655, etc. propose water-less lithographic printing plates with a photo-dimerized photosensitive layer and a silicone rubber layer as an ink repellent layer laminated on a substrate.
Furthermore, as negative type photosensitive lithographic printing plates, Japanese Patent Publication (Koho) SHO 61-616 and 61-54218, etc. propose water-less lithographic printing plates with a photosensitive layer containing a quinonediazide compound formed on a substrate and with a silicone rubber layer as an ink repellent layer formed on the photosensitive layer through an adhesive layer, and Japanese Patent Publication (Koho) SHO 61-54222, etc. propose water-less printing plates with a silicone rubber layer as an ink repellent layer formed on a photo releasable photosensitive layer. These printing plates are known to have practically excellent performance. Among them, especially Japanese Patent Publication (Koho) SHO 61-54222 proposes a negative type water-less lithographic printing plate, in which a photo releasable photosensitive layer obtained by crosslinking a partially esterified product of 1,2-naphthoquinone-2-diazido-5-sulfonic acid chloride and a phenol novolak resin by a polyfunctional isocyanate is formed on a substrate, with a silicone rubber layer as an ink repellent layer formed on the photosensitive layer.
Moreover, as photosensitive lithographic printing plates using a fluorine resin as the ink repellent layer, Japanese Patent Laid-Open (Kokai) HEI 2-254449 and 2-85855, etc. disclose water-less lithographic printing plates with a fluorine resin obtained by using 1H,1H,2H,2H-heptadecafluorodecyl acrylate or 1H,1H,2H,2H-heptadecafluorodecyl methacrylate as the ink repellent layer.
However, these photosensitive lithographic printing plates are relatively hard and fragile in the photosensitive layer, and are liable to be damaged by the stress acting on the plate surface during offset printing. According to the increase in the number of printed sheets, the photosensitive layer is damaged in the non-image area under the ink repellent layer, and the damage expands into the ink repellent layer, to lower image reproducibility disadvantageously. This appears as a problem of low printing durability of the printing plate.
To improve the printing durability, various studies have been conducted. Japanese Patent Laid-Open (Kokai) HEI 5-53306 and 5-53307, etc. propose water-less lithographic printing plates with a soft primer layer containing a natural protein and an urethane elastomer, or a polyurethane and a silane coupling agent, between the substrate and the photosensitive layer. In either case, no practically sufficient printing durability can be obtained, and since the primer layer is required to be thick enough to ensure the physical properties required for the photosensitive layer, the primer layer may be insufficiently cured to cause the peeling of the ink repellent layer and the peeling of the photosensitive layer, and furthermore in a worse case, the peeling of the primer layer itself from the substrate.
There have been attempts to increase the thickness of the ink repellent layer, but they lower the developability and lessen the ink mileage disadvantageously.
Japanese Patent Laid-Open (Kokai) HEI 1-154158 and 1-154159, etc. adopt a thicker silicone rubber layer as the ink repellent layer, and an ink retaining material is embedded for adjustment of cell depth to compensate for the decrease of the ink mileage caused by the thickening. However there remains the problem of lower developability, and adding a new step of embedding an ink retaining material, etc. is practically inconvenient. Japanese Patent Laid-Open (Kokai) HEI 1-161242 proposes a water-less lithographic printing plate prepared by exposing a plate with an ink retaining layer (photosensitive layer) at the top to an image, developing, coating it with an ink repellent layer (silicone rubber layer), and developing again. However, the plate obtained by this technique is poor in the ink retainability of the image area, and unpractically requires many steps such as the application of the silicone rubber layer after development of the photosensitive layer and twice development per plate.
The improvement of the physical properties of the silicone rubber layer as the ink repellent layer is also studied. The addition of a filler and the increase of the molecular weight of polydimethylsiloxane are proposed. Furthermore, Japanese Patent Laid-Open (Kokai) HEI 2-32349 proposes a water-less lithographic printing plate with a microporous layer containing a hardened ink repellent material, and Japanese Laid-Open (Kokai) HEI 2-8847, a water-less lithographic printing plate with a silicone rubber layer containing a graft polymer with a polyorganosiloxane as grafts. However, though these techniques can improve the flaw resistance, they cannot improve the printing durability sufficiently. On the contrary, there is a problem that they lower the ink repellency which should be kept by the silicone rubber layer.
Japanese Patent Laid-Open (Kokai) SHO 63-213848 proposes a photosensitive layer containing an acrylic acid derivative copolymer, but the proposal has problems that if the amount of the acrylic acid derivative copolymer contained in the photosensitive layer is 50 wt % or more, the image reproducibility and the adhesiveness to the silicone rubber layer as the ink repellent layer are impaired, and that if 50 wt % or less, the printing durability is insufficient.
Japanese Patent Laid-Open (Kokai) HEI 3-20741 proposes to let the photosensitive layer contain a high polymer with a vinyl carboxylate as a component, and Japanese Patent Laid-Open (Kokai) HEI 3-68946, to let the photosensitive layer contain a hydroxyphenylmethacrylamide derivative copolymer. Both the proposals state that plates which allow development by an aqueous developing solution and are excellent in printing durability can be obtained. However, these plates have a problem that since they are insufficient in resistance against the solvents of plate cleaner, UV ink, etc., the image areas are destroyed during printing, while the photosensitive layers are eroded by solvents in their non-image areas, to lower the printing durability.
The present invention has been devised to improve the disadvantages of these conventional techniques, and presents a water-less lithographic raw plate greatly improved in printing durability without being lowered in plate developability, image reproducibility, printing characteristics, solvent resistance, etc. by letting the photosensitive layer contain a compound with a specific structure, to soften the photosensitive layer, and by specifying the flexibility of the photosensitive layer and/or the primer layer in reference to the tensile properties such as initial elastic modulus, 10% stress and rupture elongation.